1. Field of the Invention
The present invention relates to a method for forming a silicon nitride film used as, for example, a protective film for semiconductor chips or memory disks and an X-ray transmission film. More particularly, the present invention relates to a method for forming a silicon nitride films with a highly controlled internal stress.
2. Description of the Prior Art
Conventionally, the CVD method and the sputtering method have often been used for formation of a silicon nitride film. The silicon nitride film formed by the CVD method, however, has had the following drawbacks. Firstly, there are used, as the raw material gases, a silicon compound such as a silicon hydride (e.g. silane SiH.sub.4), a silicon fluoride (e.g. SiF.sub.4) or a silicon chloride (e.g. SiCl.sub.4), ammonia (NH.sub.3) and nitrogen (N.sub.2); that is, the easily decomposable raw material gases contain not only silicon and nitrogen which are constituent elements of silicon nitride (Si.sub.x N.sub.y) but also other elements; consequently, the silicon nitride films formed by the CVD method inevitably contains impurities in principle. The silicon nitride films containing impurities besides silicon and nitrogen largely vary in internal stress depending upon the content of the impurities. In order to precisely control the impurities content in the silicon nitride films, it is necessary to always make the film formation (deposition) conditions constant; that is, in the thermal CVD method, for example, it is required to always make constant the deposition temperature, gas composition, gas flow rate and gas pressure. In the plasma CVD method, not only the deposition temperature, gas composition, gas flow rate and gas pressure but also the plasma state must be made constant. To always keep these parameters constant is extremely difficult and the impurities content in the silicon nitride cannot be kept constant. Thus, the precise control of internal stress of silicon nitride films has been impossible in the CVD method. Secondly, the impurities in the silicon nitride films significantly reduce the chemical stability of the films. For example, in the substrate dissolution step in the production of an X-ray lithography mask using a silicon nitride film as an X-ray transmission film, the partial dissolution of the silicon nitride film takes place, thereby allowing the film to have flaws. Further, the impurities in silicon nitride films are easily eliminated by the application of an ionizing radiation, thereby causing a change in the composition, optical transparency and physical properties of the film.
Meanwhile, the conventional sputtering method for forming silicon nitride films has had the following problems. Firstly, in the conventional sputtering method, the internal stress of the silicon nitride films is controlled by the pressure of the sputtering gas used. In this case, the precise control of the internal stress is impossible because the internal stress is greatly changed even by the slight change of the gas pressure, and the control of the internal stress has been possible only in the order of, for example, about 10x10.sup.8 dyn/cm.sup.2. Secondly, the gas pressure must be fairly large (at least 10 Pa) in order for the internal stress to be a tensile stress; use of a large gas pressure incurs trapping of impurities (e.g. hydrogen, oxygen) in silicon nitride; these impurities significantly reduce the chemical stability of the silicon nitride film obtained. For example, in the substrate dissolution step in the production of an X-ray lithography mask using a silicon nitride film as an X-ray transmission film, partial dissolution of the silicon nitride film takes place, thereby allowing the film to have flaws. Further, the impurities in silicon nitride films are easily eliminated by the application of an ionizing radiation, thereby causing a change in the composition, optical transparency and physical properties of the films.
As stated above, in conventional methods for forming a silicon nitride film, it has been very difficult to control the internal stress of the film.